Modulation of magnetron oscillator



1942' c. w. HANSELL MODULATION OF- MAGNETRON OSCILLATOR Filed Nov. 21, 1939 OUTPUT INVENTOR CZARENCF 1*?MNSEL L BY M ATTORNEY Patented Aug. 4, 1942 MODULATION OF I 2,292,073 MAGNETRON OSCILLATOR Clarence W. Hansel], Port Jefferson,

N. Y., assign-' or to Radio Corporation of America, a corpora- 7 tion of Delaware- Application November 21, 1939, Serial No. 305,413 1 Claim. (Cl. l79-'-'17l.5).

This invention relates to a new and improved means whereby amplitude modulation of oscillations produced in a magnetron is accomplished with little or no accompanying frequency modulation.

Amplitude modulation of magnetron oscillators has been accomplished heretofore by varying the potential between the anode and cathode thereof in accordance with signals. This amplitude modulation is accompanied by frequency modulation because the frequency of operation of the magnetron is determined in a large part by inter-electrode potential. To prevent this undesired frequency modulation, systems have been devised wherein the anode potential modulation is accompanied by corresponding magnetic field modulation in such a sense as to compensate or correct the undesired frequency modulation. In general, an increase in inter-electrode potential will increase the amplitude of output and decrease the frequency of the output by decreasing the space charge around the cathode and so lengthening the electron paths. Compensation of frequency modulation by field modulation is in a sense to tend to increase the frequency to thereby correct any tendency of the frequency to decrease on increase of modulating potential on the anode.

I have provided-a new and improved magnetron oscillator and amplitude modulator using anode potential variation which due to the arrangement produces amplitude modulation and substantially prevents frequency modulation of the oscillations from taking place during the amplitude modulation process.

In describing my invention more in detail, reference will be made to the attached drawing wherein the single figure illustrates a magnetron modulator of the amplitude-modulation type arranged for differential anode modulation with unequal anode direct-current excitation. The differential but unequal modulation of the anode potentials results in amplitude modulation of the produced oscillations, but, at the same time, tends to prevent frequency modulation of the oscillations produced.

In the drawing i indicates an electron discharge device having a split anode comprising .a pair of similar semi-cylindrical electrodes l2 and I4 surrounding a heated filament cathode IS. The tube is placed in a magnetic field having its axis parallel to the filament IS in the usual manner. The magnetic field is produced by winding I! arranged around the tube structure and connected with a source H of direct- 5 5 current potential.

cathode l5 and the anodes cans in the form of a variable resistance P is provided for adjusting the field intensity. The anodes l2 and H are connected in a high-frequency circuit which includes the conductors 20 and 22 and the radio-frequency by-pass condensers CR. When suitable direct-current potentials are applied between the i2 and I4 and the tube is subjected to a correct value of magnetic field with its axis parallel to the filament I5, oscillations are produced in the tube in a well known manner. These oscillations, the frequency of which may be a function of the circuit constants comprising conductors 20, 22; etc., or of the transit time of the electrons between the filament i5 and anodes i2 and I4 or of both, are impressed on the output circuit 24 inductively coupled to the tube structure and conductors 20 and 22.

Potentials for the electrodes I 2 and ii are supplied from a direct-current power source 30 by way of the secondary windings of a modulation potential transformer T. It is noted that the direct-current potentials supplied to one of the anodes, such as, for example, [2, is supplied through a potential varying device such as a variable resistance shown at 32. This means permits the application of unequal direct-current potentials to the electrodes i2 and II relative to the filament IS. The biasing resistance 32 is shunted by a. by-pass condenser CM of sufllcient size to by-pass potentials of the modulation potential frequency. Modulating poten tials are supplied to the primary winding of the transformer T and from the sections of the secondary winding in phase opposition on the electrodes l2 and I4.

The difl'erential modulation varies the phase relations of the electron currents. between the filament and the two anodes differential thereby varying the efilciency of conversion and the amplitude of the oscillatory energy output of the high-frequency oscillator. However, since the modulating potentials are differentially impressed on the anodes, the tendency of the frequency of the oscillations produced to decrease because of increase in the. amplitude of the potential on one anode is compensated or corrected by the tendency of the frequency of operation to increase due to the simultaneous decrease in the potential produced on the other anode. Thus, the ratio of amplitude modulation with respect to frequency modulation, in the highfrequency output from the magnetron oscillator, is very greatly improved.

tion with the modulation pliflers, sistances such 'as Thyrite (a baked silicon carbide x and clay mixture), cuprous oxide, etc.,-are a1- In operation of the device shown in the figure, the variation in amplitude of the high-frequency output, in response to input modulation potentials, may notbe strictly proportional, especially it a high degree of modulation is attempted. Consequently, I contemplate using amplitude or wave form distorting arrangements in connecinputin order to compensate for distortion in the. modulation response of the magnetron oscillator arrangement. Such distorting arrangements, employing curvature in the response characteristics of vacuum tube amor employing automaticallyv variable reready well known in the art.

What is claimed is: A double-anode magnetron oscillator having a said resistor for by-passing cathode-anode circuit including a common direct current power source and two inductive branches, each leading from the most positive terminal of said source to a respective one of the anode segments, an amplitude modulation source coupled to both of the inductances in said branches, capacitive means for lay-passing alternating currents generated by said oscillator around said power source and said inductances, means including a resistor in one of said branches for causing the potef tial 01 said power source to be applied unequally to the two said anode segments, means including a capacitor in shunt with modulation potentials around said resistor, and a utilization circuit coupled to said cathode-anode circuit.

CLARENCE W. HANSELL. 

